The STAR Detector tracks and analyzes particles such as protons, neutrons, and pions. Credit: Brookhaven National Laboratory
An experiment at the Brookhaven National Laboratory (BNL) in the United States provides new insights into the strong interaction that holds neutrons and protons together inside atomic nuclei1. Researchers at the BNL accelerated gold ions up to 99.99% of the speed of light. The ions collided with each other from opposite directions inside a specific chamber of the Relativistic Heavy Ion Collider.
The collisions break down neutrons and protons inside the gold nuclei, and freeing quarks and gluons. This forms a dense and hot state of matter called the quark-gluon plasma for a brief time.
The team, which included physicists at the Variable Energy Cyclotron Centre in Kolkata and the Panjab University in Chandigarh, used STAR, a massive detector designed to detect thousands of particles produced by each collision. Besides quarks and gluons, they identified two species of short-lived mesons with a surprising pattern of global-spin alignment. The spin alignment for one meson was unexpectedly large, whereas that for another was consistent with zero.
Each meson is made up of a quark-antiquark pair. They are intermediate particles and quickly decay into electrons and other particles.
The mesons with a large spin alignment are thought to mediate the strong interaction inside atomic nuclei. This is akin to photons’ roles in electromagnetic interaction. The mesons are active at a short distance, whereas photons operate at a long distance.
The quarks in motion can produce an effective meson-mediated field just as an electric charge in motion can generate an electromagnetic field. The researchers say that these findings will play pivotal roles in describing nuclear structure and nuclear matter.
